1. Field of the Invention
The present invention relates to a telephone apparatus used for a telephone system in which the telephone apparatus is connected to a remote destination telephone through telephone circuits after a subscriber's identification number (ID number) has been transmitted to the telephone company. In the telephone apparatus of the above type, the ID number (being recited) and the destination telephone number (being dialed) are usually written in a memory for registration and read from the memory for transmission to the telephone company.
2. Description of the Prior Art
In the United States of America, for instance, it is possible to pay a telephone charge of an automotive telephone set through a system referred to as "Calling Card" or "Card Call". In this system, the telephone company can demand payment from a telephone subscriber in a lump after having checked all the telephone charges. In more detail, when a subscriber operates push buttons or dial keys arranged on the automotive telephone set to enter a name of the telephone company, a remote destination telephone number and an ID number of the subscriber himself, the signals representative of the entered data are transmitted by air to the telephone company. When the subscriber is determined to be registered, the telephone circuits are connected between the telephone set specified by the ID number and another remote destination telephone set to be dialed and the resultant telephone charge is calculated. In these systems, however, it is very troublesome to enter a destination telephone number of ten digits and an ID number of fourteen digits, for instance, whenever the subscriber wants to call someone. Therefore, the destination telephone numbers and the ID number are usually stored in a memory unit of the telephone set by use of dial keys and a STORE key. Once these telephone numbers and the ID number are stored, since it is possible to read these data whenever necessary by simply operating the dial key or other keys two to four times, the operation of the telephone set is markedly simplified without entering the multi-digit numbers each time the subscriber calls someone.
In the above system, however, there still exist shortcomings. The basic problem is that another person near the subscriber might observe the ID number displayed on the display unit of the telephone set and use the ID number for a wrongful purpose without permission from the subscriber. This is because the ID number and/or a destination telephone number of many digits are displayed on the display unit of the set whenever these numbers are stored for registration or read for calling someone. In addition, when the telephone set is used, the ID number is not only displayed but converted into a DTMF (Dual Tone Multi-Frequency) signal for transmission of the ID number to the telephone company. Since the DTMF signal is reproduced into sound corresponding to the signal through a speaker of the subscriber's telephone set, another person near the subscriber's telephone set might hear and decode the DTMF signal for an unauthorized purpose.